The present invention relates to a process and apparatus for producing steel sheets provided with molten metal coatings by the continuous hot-dip plating method. More specifically, the invention relates to a process and apparatus for producing coated steel sheets in which the sedimentation of dross or impurities that are formed and suspended in the melt during hot-dipping is effectively promoted to yield coatings of good quality without dross defects.
The conventional continuous hot-dip plating method for producing steel sheets coated with molten metal (referred to below as molten metal coated steel sheets) consists of applying a preliminary treatment to a steel sheet through a continuous furnace, dipping the steel sheet in a plating bath, and passing it through the bath. A brief description of this method is given below with molten zinc as an example of a molten metal.
FIG. 1 is a schematic diagram showing the conventional production process. As shown, a plating tank 1 is continuously supplied with a steel sheet 2 via a snout 6 after the sheet 2 has been subjected to a surface activation treatment in a continuous furnace (not shown). The steel sheet 2 is passed around a sink roll 3 submerged in the plating bath so that it ascends through the bath and passes between snap rolls 4 and is subsequently drawn out of the bath.
When a molten metal is molten zinc, this process is called hot galvanizing. In this process of hot galvanizing, Fe dissolving out of the steel sheet combines chemically with Al and Zn to form dross (or impurities) principally composed of FeZn.sub.7 and Fe.sub.2 Al.sub.5 as indicated by dots in FIG. 1. Generally speaking, FeZn.sub.7 deposits as bottom dross 5 on the bottom of the plating tank, whereas Fe.sub.2 Al.sub.5 floats as top dross 9 on the surface of the melt in the plating bath. In actual operations, the rotation of the sink roll 3 and other submerged members creates currents, which cause part of the bottom dross 5 to become suspended in the plating bath or melt as indicated by arrows in FIG. 1.
Top dross 9 floats on the surface of the melt and can be removed with a screen, which is the most common method currently used in practice. On the other hand, the dross suspended in the plating bath is difficult to remove and unavoidably adheres to the surface of the steel sheet, thereby reducing the quality of a molten metal coating. In an extreme case, the coated steel sheet has to be discarded as scrap because of unacceptable quality.
However, the very nature of the continuous hot galvanizing method makes it impossible to prevent the formation of dross. Instead, recent efforts in the plating industry are directed to reducing the suspension of dross due to the currents created by the rotation of the sink roll and other submerged members. To this end, the depth of the plating tank is sufficiently increased to reduce the adverse effects of the currents on the bottom dross. This approach has proved reasonably successful in reducing the suspension of dross.
Increasing the depth of the plating tank is indeed effective in reducing the suspension of dross, but on the other hand, a greater amount of plating bath is necessary and the energy costs for the maintenance and control of the bath increase accordingly, as does the operational difficulty in removing the bottom dross. Another problem with this approach is that it cannot be applied to existing plating tanks, which must be reconstructed at significant cost to increase their depth.